Counterbalancing systems for sectional doors have been employed for many years. Common examples of such sectional doors are the type employed as garage doors in homes, commercial and utility buildings, and similar applications. Counterbalancing systems originally solved the need for providing mechanical assistance in the instance of very large doors for commercial installations and smaller garage doors for residential use, which were normally constructed of heavy, relatively thick wood or metal components. More recently, counterbalancing systems have been increasingly used to permit opening and closing operations by a single person and to facilitate the use of electric motors, preferably of limited size, to power the opening and closing of such doors.
Most such counterbalancing systems utilize drums which carry cables attached to the garage door. Commonly the drums are mounted above the frame defining the door opening, with a drum positioned at each end of the door such that the cables may be conveniently connected proximate the lower lateral corners of the garage door. Basically, the door is moved toward the dosed position, blocking the door opening due to gravity acting on the door as it moves from a substantially horizontal, open position above and inwardly of the door frame to a closed position. The path of the door in opening and closing is commonly defined by a track arrangement which interacts with rollers attached to the various sections of the door. The cable drums are classically interconnected with springs in a wide variety of ways so that they are progressively loaded as the door is lowered to prevent uncontrolled descent of the door and employ stored energy to assist in raising the door during subsequent opening operation.
The prevailing type of counterbalancing system for garage doors for homes normally having a seven-foot high door involves the utilization of torsion springs mounted on a shaft which is coaxial with or mounts the drums. In such systems, it is established practice to utilize cable drums having a diameter of approximately 31/2 inches to 4 inches. A torsion spring or springs mounted outwardly of the shaft has a diameter normally in excess of 11/2 inches to maintain an appropriate spring index. The drums and spring are normally mounted on a tubular shaft having a diameter of approximately 1 inch, which holds the springs and transmits torque from the springs to the drums which are attached to the tubing.
These conventional torsion counterbalancing systems require that the tube mounting the drums be positioned above the horizontal track of the door to permit raising the door as high in the door opening as possible to accommodate higher vehicles and to otherwise make optimum use of the door opening. With a counterbalancing system thus positioned and employing conventional 31/2 to 4-inch cable drums, there is a requirement that there be a minimum of 13 to 14 inches above the door opening as overhead clearance to permit the mounting of these counterbalancing systems. However, a disadvantage of these conventional systems is the increasing requirement for a counterbalancing system which can be installed in a structure having a lesser overhead clearance. Frequently, construction parameters dictate a lower ceiling within a garage or the use of beams, supports, or other objects which do not provide the necessary headroom clearance of 13 to 14 inches required for the utilization of these conventional counterbalancing systems.
In an attempt to accommodate the requirements for decreased overhead clearance, efforts have been made to modify these conventional counterbalancing systems. If the drums and tube with the mounted springs are merely moved downwardly, one or more of these elements interfere with the door during its opening and closing motion. One alternative which has been employed to solve reduced headroom requirements is to move the drums outboard or laterally of the tracks and lowered to a point that the springs and center bracket supporting the tube normally substantially medially thereof will just permit door clearance. This configuration, however, has serious limitations in that the cable binds the door to some extent due to the outward force applied during operation, and such is only effective to minimally reduce headroom clearance to a distance on the order of 12 inches.
A more drastic alternative to obtain additional headroom contemplates the movement of the entire counterbalance system to the rear of the horizontal track, i.e., inwardly of the garage to a position proximate the extremities of the horizontal track where the top of the door reposes when it is in the open position. In systems of this nature, it is necessary to route the cable by pulleys from the counterbalance system to the door frame and then to the door. Systems of this type have proven to be both inefficient and costly, while introducing a relatively large, unsightly mechanism centrally of a garage.
The aforedescribed conventional torsion spring counterbalancing systems also have the disadvantage that the weight of the spring members is such as to require the use of a support bracket which normally suspends the tubular shaft substantially medially between the drums. The stationary support bracket is also commonly employed as the stationary anchor for the torsion springs. The support bracket is attached to the door header or more commonly a special spring pad located on the garage wall thereabove. Since the stationary anchor associated with the support bracket undergoes torsional loading equal to the weight of the door, there is a constant potential for operational failure or damage and injury to installation and maintenance personnel. The torsional forces can also result in a loosening of the support bracket, loosening of the stationary spring anchor, a failure of a door opening header or spring pad, all of which can result in a quick and violent untensioning of torsion springs, thereby presenting the potential for damage or injury to any proximate objects.
Another disadvantage of such conventional torsion spring counterbalancing systems is the susceptibility to variations in balance of the door. With a drum diameter of approximately 4 inches, the drums revolve approximately seven times during an opening cycle of a 7-foot high door. As spring tension is lost through aging or extensive use, a highly noticeable variation in balance of the door is produced, as contrasted with systems which might have a lesser drum diameter and, therefore, rotate a greater number of times during opening and closing, such that the loading effect on a door is less for a given variation in spring tension. This same consideration makes it difficult to adjust the conventional 4-inch drum systems, since minute adjustments in spring tension can produce a substantial effect on a door.